On a number of occasions I have taken out my Quadratec front driver side mat and noticed moisture the size of my fist. I don't think it is coming from between the mat and the carpet. The carpet is also damp which leads me to believe it is coming from underneath. It is above and right of the plastic hook that the mat connects to. I called the dealer and he recommended pulling the door sil and looking under the carpet. I tried to but really didn't see much. He said I should pull the carpet a bit and put baby powder and wait and see if it is coming from somewhere and settling there as the powder would be hard. He also mentioned something about the seals sometimes going bad. I had the vehicle undercoated a number of months ago though by them. Anyone ever hear of such a thing? What could this be? Obviously I don't want any mold or mildew to build from this. I asked him what would it run if it was a seal. He said about $150. What seal would he be talking about? Thanks.

Enough water comes in from opening the door in the rain.
Even though your carpet may feel dry it is still damp down in the pad. The floor mat does not allow the carpet and under pad to dry. Even a little moisture will show up on the mat from condensation. Water was on the bottom of my mat in the same spot. Even though the carpet felt dry, when the mat was in it would have the damp spot on bottom after driving.
I had to drive it for a time with the mat out for it to dry completely. So whenever I open the door in the rain or whenever water drips in I remove the mat for a day or two to be sure it dries.

__________________It ain't easy being cheesy:

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It can also be condensation. The hot exhaust underneath can create condensation if the air is humid, causing the carpet to be damp. A number of members have reported this, although I've never seen it/experienced it.